1. Field of the Invention
The present invention relates to a method of controlling motorized window shades, and more specifically, a method of calibrating a motorized roller shade in order to determine a radius of the roller tube and a thickness of the shade fabric.
2. Description of the Related Art
Motorized roller shades include a flexible shade fabric wound onto an elongated roller tube. The roller tube is rotatably supported so that a lower end of the shade fabric can be raised and lowered by rotating the roller tube. The roller tubes are generally in the shape of a right circular cylinder having various lengths for supporting shade fabrics of various width. Motorized roller shades include a drive system engaging the roller tube to provide for tube rotation. The shade fabric is typically moved between an open position and a closed position.
For aesthetic reasons, it is desirable that the outer diameter of the roller tube be as small as possible. Roller tubes, however, are generally supported only at their ends and are otherwise unsupported throughout their length. Roller tubes, therefore, are susceptible to sagging if the cross-section of the roller tube does not provide for sufficient bending stiffness for a selected material. Therefore, an increase in the length of a roller tube is generally accompanied by an increase in the outer diameter of the tube.
In certain situations, such as for shading areas of very large width or for shading areas that are non-planar across their width, it may be desirable to use multiple roller shades. In these situations, it may also be necessary or desirable to use roller tubes having different lengths. Relatively long tubes might require that a larger diameter be used compared to shorter tubes in order to limit sagging. Where multiple roller shades are used to shade a given area, the capability of raising or lowering the shades such that their lower ends move consonantly as a unit (i.e., simultaneously at the same speed) is desirable. However, two roller shades having tubes of differing diameter will not raise or lower a shade fabric at the same speed if they are rotated at the same rotational speed.
For any member that is rotated about a central axis, the linear speed at a surface of the rotating member will depend on the distance between the surface and the rotational axis. Thus, for a given rotational speed (i.e., rpm), the resulting linear speed (i.e., in/sec) at the outer surface of the tube will vary in direct proportion to outer tube diameter. Therefore, two roller tubes having differing outer diameters that are driven at the same rotational speed will have different linear speeds at the outer surface. The larger diameter tube will have a higher linear speed at the outer surface and, accordingly, will windingly receive, or release, the associated shade fabric at a faster rate than the smaller diameter tube.
The ability to provide constant shade speed for two roller shades having tubes of differing diameters is further complicated because the shade speed for either one of the roller shades will not remain constant as the shade is raised or lowered between two shade positions. The winding receipt of a shade fabric onto a roller tube creates layers of overlapping material that increase the distance between the rotational axis and the point at which the shade fabric is windingly received compared to the distance at the tube outer surface. As a result, the shade speed will vary depending on the thickness of the overlapping layers of material received on the roller tube.
A prior art motorized window treatment control system provides a method for controlling shade fabric speed for multiple motorized roller shades to achieve a constant linear speed of the hembar (i.e., the lower edge of the shade fabric). The desired linear shade speed, roller tube diameter, shade fabric thickness, and shade fabric length are stored in a memory for use by a microprocessor of the motorized window treatment controller. Preferably, the roller tube rotational speed is varied by the microprocessor depending on shade position determined by signals from Hall effect sensors. The microprocessor maintains a counter number that is increased or decreased depending on direction of rotation. Based on the counter number, the microprocessor determines shade position and a corrected rotational speed for the desired linear shade speed. Preferably, the microprocessor controls roller tube rotational speed using a pulse width modulated signal. The system may be used to control first and second roller shades having roller tubes of differing diameters or shade fabrics of varying thicknesses. The method for controlling the linear speed of a roller shade fabric is called Intelligent Hembar Alignment (IHA) and is described in greater detail in commonly-assigned U.S. patent application Ser. No. 10/774,919, filed Feb. 9, 2004, entitled CONTROL SYSTEM FOR UNIFORM MOVEMENT OF MULTIPLE ROLLER SHADES, the entire disclosure of which is incorporated herein by reference.
The inputs of the method of controlling the linear speed of a roller shade fabric, i.e., roller tube diameter, shade fabric thickness, and shade fabric length, are often not known at the time of installation and configuration of the control system. It is preferable to program these values of the roller tube diameter, shade fabric thickness, and shade fabric length in the memory of the microprocessor of the motorized window treatment controller before being shipped. However, this requires that a production worker measure the roller tube diameter and the shade fabric thickness with a measuring tool, such as a pair of calipers, at the time of manufacturing. Accordingly, this increases the time required for the manufacturing process and increases the cost of the motorized rollers shades.
Further, there are some installation factors that usually cannot be determined at the time of manufacturing, but still affect the values of the roller tube diameter, the shade fabric thickness, and shade fabric length. For example, the initial wrap (i.e., the amount of shade fabric that is wrapped around the roller tube when the shade fabric is in the closed position) is not typically known at the time of manufacturing. Variation of the amount of initial wrap results from variation in the mounting height of the roller shade at the time of installation.
What is needed, therefore, is a method of calibrating a roller shade in order to quickly determine the radius of the roller tube and the thickness of the fabric such that the linear speed of the roller shade can be easily controlled.